Pasture utilization by free-ranging cattle in Central Australia : a case study

Abstract

In Central Australia, the need exists for a study of pasture utilization by free-ranging cattle at increasing distances from stock watering points. Two of the most important pasture types, the Mitchell Grass plains and the Open Woodland eco-units were chosen for the study. It was expected that the effects of grazing would be less obvious with increasing distance from watering points, where grazing pressure was assumed to be minimal. Also, by assessing the degree of utilization of the major grasses, the valuable pasture species could be identified.

Sampling to determine basal cover, pasture composition and utilization of pasture species, was carried out at regular and increasing distances from relevant watering points. This involved the implementation of the wheel-point method of botanical survey at h, 1» 2, 3, 4, 5, 6 km. distance from the focal waters. Analysis was undertaken during July, August, 1977 along 3 radii from Limestone Bore, Amburla Station, in the Mitchell Grass community, and a single radius from Brumby Bone, Milton Park, in the Open Woodland eco-unit. The appropriate site sample size in both pasture types surveyed was based on earlier pilot studies.

Employing the concept of the equivalent fully-grazed-tuft, an index of utilization for pasture species at a site was devised. This was further extended to the development of a "relative use value" which represents the contribution of various plant species to the diet of cattle at any one site. Based upon this index, a species preference rating was derived which relates the amount of forage available to the amount used. This made it possible to identify the valuable or key forage species.

It was evident from an analysis of the Limestone Bore data that the effects of grazing and utilization of pasture species decreased as grazing pressure decreased away from the watering point. Heavy utilization of pastures at the time of survey appeared to be concentrated within a 3 to 4 km. radius of the bore. At 6 km. from the bore there was virtually no grazing effect. The valuable forage species were identified as Mitchell grass (Astrebla pectinata) and Oat grass (Enneapogon avenaceus).

An analysis of the Brumby Bore data revealed the influence of a highly preferred pasture type on grazing concentrations. A small section of Cottonbush floodplain merged with the single radius sampled at Brumby Bore at approximately the 2 to 3 km. distance from the bore. It was found that intense grazing was not only centredat the watering point, but also around this highly-preferred pasture type. The effects of grazing and utilization of pasture species decreased away from this region. Very little grazing influence was noted at 5 km, distance from the bore. The importance of other foragers, particularly kangaroos, was also observed in this pasture type. The valuable forage species were identified as once again Oat grass, and also Umbrella grass (Digitaria coenicola).